Improving the ability of a railway car to smoothly travel along a curved track is and has been an important technical problem. There is a strong desire for an increased ability of a railway car to travel along curves, particularly for railway cars traveling along sharp curves in suburban railways such as underground railways.
FIG. 14 is an explanatory view schematically showing the behavior of a conventional truck 3 in which the wheels are not steered with respect to a truck frame 2 when traveling along a curved track 4. The truck frame 2 which is traveling along a curved track 4, the wheelset 1f positioned to the front in the direction of travel (referred to in this description as the front wheelset) and the wheelset 1r positioned to the rear in the direction of travel (referred to in this description as the rear wheelset) assume the attitudes shown in FIG. 14. Symbol O in FIG. 14 indicates the center of the arc defined by the curved track 4.
Non-Patent Document 1 discloses that (a) the flange of the wheel 5 on the outer side of the front wheelset 1f contacts the rail 4a on the outer side and an attack angle θ develops; (b) this attack angle θ causes a lateral pressure Qsi to be applied by the inner track; and (c) the rear wheelset 1r is located approximately midway between the left and right rails 4a and 4b, so in the rear wheelset 1r, an attack angle θ does not develop to the same extent as in the front wheelset 1f. However, since a sufficient difference between the rolling radius of the left and right wheels 5 is not obtained, the radius difference in the rear wheelset is insufficient and causes a longitudinal creep force Fvc to develop. The inner track lateral pressure Qsi and the longitudinal creep force Fvc produce a yawing moment My in the counterclockwise direction about the center of gravity of the truck frame 2. In FIG. 14, Qso indicates the outer track lateral pressure which develops in the front wheelset 1f. 
Non-Patent Document 2 discloses that the truck frame 2 also has a yawing angle φ which is defined as the angle in a horizontal plane of the truck frame to the left and right with respect to the radial direction of the curved track. The yawing angle φ of the truck frame 2 has the same rotational direction as the attack angle θ of the front wheelset 1f. The yawing angle φ of the truck frame 2 causes the attack angle θ of the front wheelset 1f which is supported by this truck frame 2 to further increase.
Patent Document 1 discloses an invention in which in order to increase the ability of a railway car to travel along a curved track, an actuator is used as a supplemental means so that the truck frames which are positioned to the front and rear in the direction of travel pivot in synchrony with respect to the car body in the self-steering direction. That invention can decrease the yawing angle of the truck frame during travel along a curved track.
However, in order to carry out the invention disclosed in Patent Document 1, it is necessary to provide not only an actuator but also a controller for the actuator. In addition, it is necessary to provide safety measures for the event in which control of the actuator cannot be carried out in a normal manner. Therefore, the apparatus becomes complicated and costly.
A link-type steerable truck which uses links without using an actuator is also being developed. FIG. 15 is an explanatory view schematically showing the structure of a typical link-type steerable truck 11. FIG. 15(a) is a plan view and FIG. 15(b) is a side view thereof.
In this steerable truck 11, the front wheelset 1f and the rear wheelset 1r are connected to a bolster 12, which is mounted on an unillustrated car body, and to a truck frame 13 by pairs of first links 14a and 14b. Of the first links 14a and 14b, each of the first links 14b which is connected to the truck frame 13 (referred to below as steering levers 14b) is connected to an axle box 19 which rotatably supports the front wheelset 1f or the rear wheelset 1r by a second link 15.
In this steerable truck 11, displacement of the bolster 12 on the car body side with respect to the truck 11 by the bogie angle is transmitted to the steering levers 14b throng the first links 14a. In the example shown in FIG. 15, the connection points between the first links 14a and the steering levers 14b are connection points 16 on the car body side.
The transmitted displacement adjusts the steering amount based on the lever ratio when the connection points between the steering levers 14b and the truck frame 13, i.e., the connection points 17 on the truck frame side act as centers of pivoting (fulcrums), and the front wheelset 1f and the rear wheelset 1r are steered through the connection points between the steering levers 14b and the second links 15, namely, through the connection points 18 on the wheelset side.
FIG. 16 is an explanatory view showing the behavior of the steerable truck 11 when traveling along a curved track.
As shown in FIG. 16, in this steerable truck 11, the steering angle α1, which is the angle between the centerline CL1 of the front wheelset 1f and an imaginary straight line CL3 in a horizontal plane connecting the center of the truck frame 13 with the center of a circular arc defined by the curved track, is the same as the steering angle α2 formed between the centerline CL2 of the rear wheelset 1r and the straight line CL3.    Non-Patent Document 1: “Properties of Trucks and Tracks During Travel Along a Sharp Curve and their Effect on Rail Corrugation”, J-Rail '95    Non-Patent Document 2: “Methods of Measuring the Attack Angle of Wheels and the Relative Displacement of Wheels and Rails by Measurement on the Ground”, Proceedings of the 73rd Regular General Meeting of the Japan Society of Mechanical Engineers    Patent Document 1: JP 2002-87262 A1